The present invention relates generally to hearing screening and diagnostic techniques. More specifically, the invention provides a method and system for calibrating hearing equipments and determining hearing status. Merely by way of example, the invention has been applied to audiometer, but it would be recognized that the invention has a much broader range of applicability.
Hearing loss can be categorized by where or what part of the auditory system is damaged. There are three basic types of hearing loss: conductive hearing loss, sensorineural hearing loss and mixed hearing loss.
Conductive hearing loss occurs when sound is not conducted efficiently through the outer ear canal to the eardrum and the tiny bones, or ossicles, of the middle ear. Examples of conditions that may cause a conductive hearing loss include: conditions associated with middle ear pathologies such as fluid in the middle ear from colds, allergies, poor eustachian tube function, perforated eardrum, benign tumors disarticulated ossicles, ossification of ligament, impacted earwax, and infection in the ear canal.
Sensorineural hearing loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear (retrocochlear) to the brain. Sensorineural hearing loss can be caused by diseases, birth injury, drugs that are toxic to the auditory system, and genetic syndromes. Sensorineural hearing loss may also occur as a result of noise exposure, viruses, head trauma, aging, and tumors.
Mixed hearing loss results when a conductive hearing loss occurs in combination with a sensorineural hearing loss. In other words, there may be damage in the outer or middle ear and in the inner ear (cochlea) or auditory nerve.
Various techniques have been developed for screening hearing problem. For instance, an analysis of the acoustic power reflectance in the ear canal has been shown to be effective in diagnosing conductive hearing loss problems while an otoacoustic emission test (OAE) has been shown to be effective for diagnosing sensorineural hearing loss issues.
Acoustic power reflectance is the measurement of the amount of power being reflected from the ear drum, middle ear structure, and/or cochlea. More specifically, it is defined as the ratio of the forward-moving (incident) pressure wave to the reflected (retrograde) pressure wave. Consider an acoustic pressure wave that travels along an ear canal, as long as no discontinuities exist in the ear canal, the acoustic power that is conveyed by the pressure wave propagates unimpeded to the eardrum. The eardrum then conducts the acoustic power into the middle ear. However, due to mismatch in ear canal impedance, while some of the incident power that reaches the eardrum will enter the middle ear, the remainder is reflected back into the ear canal. The reflected power takes the form of a retrograde pressure wave in the ear canal.
The magnitude and latency of the reflected waves as a function of frequency, is a useful indicator of the status of the middle ear. Reflected acoustic power that is significantly different in magnitude or latency from that of a normal ear will likely reveal the precise nature of a disorder.
The otoacoustic emission test (OAE), on the other hand, measures the sounds that the ear produces in response to stimulation. There are two common types of otoacoustic emissions in clinical use: Transient otoacoustic emissions (TOAEs) or transient evoked otoacoustic emissions (TEOAEs) are sounds emitted in response to a train of acoustic stimuli of very short duration. These stimuli are usually clicks but can be tone-bursts. Distortion product otoacoustic emissions (DPOAEs) are sounds emitted by the cochlear hair cells in response to two simultaneous tones of different frequencies.
While the above mentioned hearing testing techniques are extensively used in the detection of hearing loss, the presence of standing wave can adversely affect calibration of audiometers and also the accuracy of these hearing tests. Therefore, it would be desirable to have hearing test methods and systems that produce result that are affected by reduced distortions from standing waves or even free from any distortion by standing waves.